The (optional) JSON-GLib library adds dependencies on GObject, GIO. For
statically linked oss-fuzz builds it also adds libffi and more. To avoid
these dependencies, replace JSON-GLib by some custom code. This allows
`tshark -G elastic-mapping` to be enabled by default without extra deps.

API design goals of the new JSON dumper library:

- Small interface without a lot of abstraction.
- Avoid memory allocations if possible (currently none, but maybe
json_puts_string will be replaced to improve UTF-8 support).
- Do not implement parsing, this is currently handled by jsmn.

Methods to open/close array/objects and to set members are inspired by
the JsonGlib interface. The interfaces to write values is inspired by
the sharkd code (json_puts_string is also borrowed from that).

The only observed differences in the tshark output:
- JSON-GLib ignores duplicates, json_dumper does not and may produce
duplicates and currently print two "ip.opt.sec_prot_auth_unassigned".
- JSON-GLib adds a space before a colon (unimportant formatting detail).
- (Not observed, but UTF-8 strings will be wrong like bug 14948.)

A test was added to catch changes in the tshark output. I also fuzzed
json_dumper with libFuzzer + UBSAN/ASAN and fixed an off-by-one error.